Within industry, adhesive tapes have long been used for producing cable looms. In this application, the adhesive tapes serve to bundle a multiplicity of electrical leads prior to installation or in the already assembled state, in order, by bandaging them, to reduce the space taken up by the bundle of leads, and also to achieve additional protective functions.
Within the motor vehicle industry, adhesive tapes for cable cladding are tested and classified according to extensive banks of standards, such as, for example, LV 312-1, “Protective systems for lead harnesses in motor vehicles, adhesive tapes; testing guidelines” (October 2009), as a joint standard from the companies Daimler, Audi, BMW and Volkswagen, or the Ford specification ES-XU5T-1A303-aa (revised version of September 2009) “Harness Tape Performance Specification”.
In widespread use are cable wrapping tapes with film carriers and textile carriers, coated generally on one side with various pressure-sensitive adhesives. As well as a range of requirements, such as chemical compatibility, high bond strength, compatibility with changing substrates, that are imposed on adhesive tapes, it must also be ensured in the motor vehicle industry that uneven, non-uniform substrates are reliably bonded by the cable runs, convoluted tubes, and branches. Other factors are flexural and tensile stresses in the course of production, installation and subsequent use within the engine compartment of a motor vehicle, or else in the vehicle body, with continual flexural stress during the opening of doors.
Since the end of the adhesive tape is ideally bonded to its own reverse face, there must be good instantaneous bond strength (tack) to this substrate, so that flagging of the adhesive tape does not occur at the start. In order to ensure a flagging-free product durably, the anchoring on the substrate and the internal strength of the adhesive must both be such that the adhesive bond is robust even under the effect of tension (tensile and flexural stressing).
Flagging—in the context of an adhesive tape wound around a body—refers to the tendency of one adhesive tape end to stand up. The reason lies in the combination of holding force through the adhesive, the stiffness of the carrier, and the diameter of the cable harness.
Determining the flagging resistance of wire harnessing (WH) cable wrapping tapes is done via the TFT (Threshold Flagging Time) method. The target variable for an outstandingly flagging-free woven fabric product is defined with a limiting value of well above 1000 min TFT, preferably above 2000 min TFT.
The realization of easy-unwind WH adhesive tapes in conjunction with retention of good technical adhesive properties poses a major challenge, since the two properties appear to be mutually exclusive: the key criteria for single-side bonding cable wrapping tapes, with adapted unwind force and sufficiently high bond strength, are in flat contradiction with one another. Whereas good flow-on behaviour and anchoring behaviour on the part of the PSA are prerequisites for good bond strength values and an associated low flagging potential, these criteria are more of a hindrance to convenient unwind performance.
EP 1 548 080 A1 discloses industrial adhesive tapes having a filler content of 10% for use in construction, the tapes being based on AcResin Polymers having molecular weight of an estimated 200 000 g/mol and also on further additions such as flame retardants. Up to 10% of fillers may be present. There is no reference to the possibility of use in the motor vehicle sector under the aforementioned ambient conditions, such as the high temperature.
EP 1 370 590 B1 (WO 02/28963 A2) discloses a process for preparing highly crosslinked acrylate hotmelt PSAs from amide-functional acrylates. Further disclosed is a process for producing a UV-crosslinkable acrylate composition.
Known under the AcResin trade name are unblended polyacrylates with molecular weights Mw of around 200 000 to 300 000 g/mol. As a result of this it is necessary to enable sufficiently high cohesion through the UV crosslinking, though at the same time there is a considerable drop in adhesion because of the high degree of crosslinking.
Specifically in relation to flagging behaviour, however, it is important to bring about a balance between cohesion and adhesion. Even at low UV doses, the unblended AcResin composition is too heavily crosslinked, resulting in a very small processing window and/or ruling out the possibility of varying the product by altering the machine parameters.
In order to make this possible, it is known for the AcResin to be blended with resins, for example.
In order to bring about the desired properties in the PSAs, the known acrylate polymers can be admixed with standardized blends. The purpose of the blended polymers is to increase the adhesion. The cohesion is not significantly improved at the same time. For that purpose, preference is given to using resins having a low softening point (rosins, for example). On account of the modulus of elasticity, which drops greatly as a result, the cohesion of the adhesive is lowered. With the objective of improving the cohesion, resins having a very high softening point are used, for example (for example, terpene-phenolic resins having a softening point >115° C.), leading to an improvement in the cohesion. While the modulus of elasticity is likewise lowered, it is nevertheless compensated by the sharper increase in glass transition temperature. At the same time, however, there is a deterioration in the flow-on behaviour, owing to the high resulting glass transition temperature by comparison with the unblended adhesive. Accordingly it becomes very difficult to ensure that the balance of adhesion and cohesion is constant over time. Such a balance is of great importance for WH cable winding tapes, especially for the setting of a stable unwind force.
The selection of the resins is likewise greatly restricted, since the majority of resins absorb in the UV range and therefore higher doses are needed in order to achieve an even degree of crosslinking. This is a disadvantage, since the higher UV dose may significantly increase the thermal input into the carrier, with the possible consequent need for extensive cooling in order to prevent the carrier contracting. On the processing side, it may be necessary to lower the belt speed in order to be able to achieve the required UV dose and/or to dissipate the thermal energy. Resins with high UV absorption may lead to very sharp crosslinking gradients, meaning that the anchoring on the woven fabric is no longer ensured.
A further possibility for modification lies in the use of fillers such as Aerosil (fumed silica), for example. With these fillers, the cohesion can be increased through filler-polymer interaction. Incorporating the fillers homogeneously is associated with a very high level of technical effort and complexity. If such homogeneous incorporation is not achieved, the benefit of the fillers is absent, since the system will then break up on agglomerates, or the specific surface area is not sufficiently great for polymer chains to be sufficiently adsorbed. Moreover, agglomerates may lead to an inconsistent or rough/streaky coating. In principle, the fillers lead to a reduction in the bond strength.